This invention generally concerns methods for producing structures over 15 microns in height from photosensitive coatings with an arbitrary surface-relief profile by scanning a pattern on the coating and then developing it. The resulting structure may, for example, be micro-optical elements such as lenses and gratings, or alignment and registration marks such as cross hairs.
Previous methods of fabrication of elements using exposures of photoresist either concentrated upon fabricating binary (two-level) structures, such as in the case of micro electromechanical systems (MEMS), or exposed photoresist to a varying dosage of exposure radiation using thin photoresist. Photoresist has been exposed to create continuous-relief photoresist profiles of optical devices (e.g., microlenses, diffractive phase plates, and diffraction gratings). Exposure methods have included laser pattern generation, grayscale mask exposures, and holography.
The present invention overcomes the limitations of such prior processes in affording arbitrary microstructure profiles, especially with heights (sag in the case of lenses) exceeding 15 xcexcm.
Laser pattern generators (LPGs) have been proposed to expose photoresist in a point-by-point fashion with variable exposure doses. Likewise, the use of grayscale mask lithography has been proposed to provide continuous-relief profiles in photoresist.
The present invention improves upon prior LPG methods even for the fabrication of diffractive elements by creating photoresist layers with uniform thicknesses over 15 xcexcm, and then patterning the photoresist layer with blazed multilevel or continuous profiles at depths exceeding 15 xcexcm. Prior to the present invention, no one had fabricated continuous-relief microlenses using a variable exposure dosage that resulted in microstructures with surface sags exceeding 15 xcexcm.
The invention is a method of fabricating a structure on a substrate using a photosensitive material in liquid form which hardens by drying. The method comprises the steps of creating a uniform layer of the photosensitive material on the substrate having a height greater than 15 microns by spinning the substrate, at least partially but not fully drying the layer by allowing it to set at ambient temperature, and then fully drying and hardening the layer by applying heat to the bottom surface of the substrate. A surface relief pattern is then produced on the hardened layer of material with a scanning beam of varying dosage of electromagnetic radiation, and the exposed portions of the layer are developed in order to produce the structure.
In the preferred method of the invention, the photosensitive material capable of being processed with low contrast such as S1075 photoresist sold by Shipley Company, LLC located in Marlborough, Mass. The substrate is spun at a sufficiently low speed to produce a layer of photoresist having a thickness greater than 15 microns. In order to achieve a more uniform thickness, the spin-coating step of the method is preferably performed initially at a lower speed (for example, 200 rpms for 30 seconds) in order to spread the photoresist uniformly on the substrate, and then at a faster speed (i.e., 500 rpms for 3 minutes) to achieve the desired final thickness of the film.
After the spinning step, the photoresist-coated substrate is allowed to dry or set at ambient temperature for preferably 30 minutes in order to reduce stresses in the thick, viscous photoresist film, and to achieve a greater uniformity of thickness. The heating step is preferably performed by a hot plate with a ramping feature so that the photoresist is slowly ramped up to a soft baking temperature, thereby reducing the chance of solvent bubbles forming in the photoresist. In the preferred method, the hot plate is ramped up to a final temperature of 90xc2x0 C., and the film-covered substrate is then baked for 10 minutes in order to completely harden the photoresist.
At all times during the spinning, air drying, and bake-drying steps, the coated substrate is maintained at a level orientation so that the photoresist layer (which is still in a plastic state) does not develop any wedging due to gravity-induced flow. The method of the invention is fully capable of creating photoresist film thicknesses of 50 microns or greater that are extremely uniform, thereby overcoming a major obstacle in the prior art to the creation of small structures in photoresist having heights greater than 15 microns. The use of a low contrast photoresist to form the thick photoresist layer allows patterns of arbitrary profile to be scanned into the layer and developed into the desired structures.